Domed tip cap and related method

ABSTRACT

A bucket for a turbine includes a dovetail mounting portion, a shank portion defined at a radially outer end thereof by a platform, and an airfoil portion extending radially outward of the platform. The airfoil portion has a radially outer end closed by a tip cap having a non-planar shape that reduces stress in the tip cap during use. A related method of reducing stress in a tip cap of a turbine bucket includes providing an airfoil portion of the turbine bucket with a radially outer end; and closing the radially outer end with a tip cap having a non-planar shape that reduces stress in the tip cap during use.

BACKGROUND

This invention relates to turbine technology and, more specifically, to cap constructions for turbine buckets or blades. Traditionally, tip caps have been used to cover the tips of airfoil portions of turbine buckets that have internal serpentine cooling circuits. These tip caps are manufactured from flat plate stock and are typically either welded to the radially outer end of the airfoil portion of the bucket, or cast in with the airfoil portion. In either case, the flat tip caps are subject to high stresses during operation.

BRIEF SUMMARY OF THE INVENTION

In one exemplary but non-limiting embodiment, a tip cap insert is adapted to be welded to the radially outer end of a bucket airfoil portion. In accordance with this embodiment, the tip cap is in the shape of an inverted dome, i.e., it is substantially concave in a radially inward direction (as seen from the exterior of the bucket).

The tip cap is also formed with a peripheral flange adapted to seat on an internal shoulder within the airfoil portion to which the tip cap is welded.

In another exemplary but non-limiting embodiment, the domed tip cap is cast integrally with the airfoil portion and other airfoil features of the bucket.

Accordingly, in one aspect, there is provided a bucket for a turbine comprising a dovetail mounting portion, a shank portion defined at a radially outer end thereof by a platform, and an airfoil portion extending radially outward of the platform, the airfoil portion having a radially outer end closed by a tip cap wherein one or both of an interior and exterior surface of the tip cap is formed of a non-planar shape that reduces stress in the tip cap during use.

In another aspect, there is provided a bucket for a turbine comprising a dovetail mounting portion, a shank portion defined at a radially outer end thereof by a platform, and an airfoil portion extending radially outward of the platform, the airfoil portion having a radially outer end closed by a tip cap wherein at least an interior surface of the tip cap is domed in a radially inward direction; and wherein the tip cap is formed with one or more internal ribs.

In still another aspect, there is provided a method of reducing stress in a tip cap of a turbine bucket comprising an airfoil portion of the turbine bucket with a radially outer end; and closing the radially outer end with a tip cap having a non-planar shape that reduces stress in the tip cap during use.

A more detailed description will now be provided in connection with the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional turbine bucket;

FIG. 2 is an enlarged detail of the tip cap portion of the airfoil shown in FIG. 1, shown at a slightly different angle, to make visible the planar tip cap;

FIG. 3 is a plan view of the airfoil portion illustrated in FIGS. 1 and 2;

FIG. 4 is a section taken along the line 4-4 of FIG. 3;

FIG. 5 is a top plan view of an airfoil portion of a bucket having a domed tip cap in accordance with an exemplary but non-limiting embodiment of the technology disclosed herein;

FIG. 6 is a partial section taken along the line 6-6 of FIG. 5;

FIG. 7 is a section similar to FIG. 6, but showing the domed tip cap cast in with the airfoil portion of the bucket in accordance with another exemplary but non-limiting embodiment; and

FIG. 8 is a section having a similar orientation as in FIGS. 6 and 7 but illustrating another tip cap shape in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIGS. 1-4, a conventional internally cooled bucket 10 for a turbine engine comprises a mounting dovetail 12, a shank or shank portion 14, a platform 16 atop the shank, and an airfoil portion or airfoil 18 extending radially outwardly from the platform. The airfoil 18 terminates at its outermost end with a tip cap 20. The tip cap illustrated in FIGS. 1-4 comprises a flat plate welded to the airfoil portion. More specifically, and as best seen in FIG. 4, the flat tip cap 20 is seated on an internal peripheral shoulder or flange 22 and welded thereto in accordance with conventional welding techniques. It will be appreciated that the flat tip cap may also be cast in with the airfoil, eliminating the need for welding. Internal ribs 24 (shown in phantom) may be employed to reinforce the tip cap as necessary.

Turning now to FIGS. 5 and 6, a tip cap in accordance with an exemplary but non-limiting embodiment of this invention is illustrated. For convenience, reference numerals corresponding to those used in FIGS. 1-4 are used in FIGS. 5 and 6 to designate common components, but with a prefix 1 added. Thus, the bucket 110 includes an airfoil 118 with a tip cap 120 secured at the radially outermost end of the airfoil. The tip cap 120 may be described as having an inverted dome shape, i.e., the tip cap is concave in a radially inward direction (or as viewed externally of the bucket). In other words, the tip cap projects radially into the interior of the bucket. The tip cap 120 is formed with a peripheral flange or edge 26 that is seated on an internal shoulder or flange 122 of the airfoil 118 and, again, welded thereto in accordance with conventional welding techniques.

In another exemplary but non-limiting embodiment illustrated in FIG. 7, the tip cap 220 of airfoil 218 is cast in with the airfoil portion and, like the tip cap in FIG. 6, has an inverted dome or inwardly concave shape.

By imparting an inverted dome shape to the (i.e., planar) tip cap, bending stresses typically associated with flat tip caps can be reduced. In some cases, the stress-reducing benefit of the dome cap may be sufficient to justify elimination of rib welds that are traditionally used to secure the tip caps to their respective airfoils. It will be appreciated that the curvature of the tip cap dome is optimized to provide stress reduction to all regions of the tip cap, regardless of the span being covered. In addition, the domed tip cap may be provided with known surface features (such as turbulators) to enhance convection (see, for example, internal ribs 122 in FIG. 3 and 124 in FIG. 5).

FIG. 8 illustrates another exemplary, non-limiting implementation where the tip cap 320 of bucket 318 is curved inwardly on its outer surface 321 while remaining flat on its inner surface 323. This shape also provides the desired stress reduction. Thus, it will be appreciated that the term “domed” is intended to embrace any number of shapes for one or both of the interior and exterior surfaces of a bucket tip cap (whether welded to or cast in with the airfoil portion of the bucket), that may include circular, parabolas, or otherwise complex shapes that deviate from the traditional flat plate stock and that reduce stress.

The dome shape of the tip cap also enables a reduction in required material capability and/or material thickness, and increased operating temperatures of the tip cap may be possible without altering the material composition. In addition, wider openings can be covered than with flat designs of the same material and thickness.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A bucket for a turbine comprising a dovetail mounting portion, a shank portion defined at a radially outer end thereof by a platform, and an airfoil portion extending radially outward of said platform, said airfoil portion having a radially outer end closed by a tip cap wherein one or both of an interior and exterior surface of the tip cap is formed of a non-planar shape that reduces stress in the tip cap during use.
 2. The bucket of claim 1 wherein said non-planar shape is domed in a radially inward direction.
 3. The bucket of claim 2 wherein said tip cap is cast-in with said airfoil portion.
 4. The bucket of claim 1 wherein said tip cap is welded to said airfoil portion.
 5. The bucket of claim 4 wherein said tip cap is formed with a peripheral flange seated on a shoulder formed in said airfoil portion.
 6. The bucket of claim 1 wherein said tip cap is formed with one or more internal ribs.
 7. A bucket for a turbine comprising a dovetail mounting portion, a shank portion defined at a radially outer end thereof by a platform, and an airfoil portion extending radially outward of said platform, said airfoil portion having a radially outer end closed by a tip cap; wherein at least an exterior surface of said tip cap is domed in a radially inward direction; and wherein said tip cap is formed with one or more internal ribs.
 8. The bucket of claim 7 wherein said tip cap is cast-in with said airfoil portion.
 9. The bucket of claim 7 wherein said tip cap is welded to said airfoil portion.
 10. The bucket of claim 9 wherein said tip cap is formed with a peripheral flange seated on a shoulder formed in said airfoil portion.
 11. A method of reducing stress in a tip cap of a turbine bucket comprising providing an airfoil portion of the turbine bucket with a radially outer end; and closing the radially outer end with a tip cap having a non-planar shape that reduces stress in the tip cap during use.
 12. The method of claim 11 including welding the tip cap to the airfoil portion.
 13. The method of claim 12 wherein the tip cap is seated on an internal flange in the airfoil portion.
 14. The method of claim 11 wherein the tip cap is cast in with the airfoil portion.
 15. The method of claim 11 wherein said tip cap is formed with one or more internal ribs.
 16. The method of claim 11 wherein interior and exterior surfaces of said tip cap are domed in a radially inward direction.
 17. The method of claim 11 wherein an exterior surface of said tip cap is domed in a radially inward direction. 